Bahrain's most prominent human rights activist has been summoned for questioning by a military prosecutor, after being accused of tampering with photographs of a man who died in custody last week.

Nabeel Rajab, the head of the Bahrain centre for human rights was accused of posting a "fabricated image" of a detainee on his Twitter account, according to the interior ministry.

Rajab claims Ali Isa Saqer was beaten to death in custody. He told the Associated Press that the photo he posted on his Twitter account, showing Saqer's body covered with bruises and gashes, was genuine. Rajab said there is a campaign to prevent him from documenting human rights abuses in Bahrain.

"They want to do their crimes in secret," Rajab said. "I am one of the few human rights activists who has not yet been arrested and the government wants to silence me and prevent me from doing my work."

Bahrain has been cracking down on critics since the imposition of military rule last month after weeks of protests by the Shia majority – who say they are treated as second-class citizens – against the Sunni monarchy.

Khawaja, 50, has worked for national and international human rights organisations, including the Bahrain Centre for Human Rights and Front Line, a human rights group in Dublin.

"The brutal beating of rights activist Abdul Hadi al-Khawaja by police during a warrantless predawn raid adds cruelty on top of illegality," said Joe Stork, deputy Middle East director at Human Rights Watch. "He should be released immediately."

According to Khawaja's daughter, Zainab al-Khawaja, about 15 masked men, armed and mostly in black uniforms, stormed into her fourth-floor apartment in the village of Muqsha, They seized Khawaja and two of his sons-in-law, Wafi al-Majid and Hussein Ahmed, to a third-floor landing where they beat and kicked them.

As for Rajab, the official Bahrain news agency and a newspaper close to the government accused him in September of being part of a "terrorist network" and of passing "false information" to international organisations for the purpose of "harming Bahrain's reputation. Later that month, he was prevented from travelling to Saudi Arabia. In December, Rajab's computer was confiscated as he was about to board a plane at Bahrain international airport. It was returned with the power on, indicating that information may have been downloaded or copied.

Meanwhile, Bahrain's public prosecutor has begun questioning three senior journalists sacked from the Gulf kingdom's only opposition newspaper over accusations that they falsified news about the government's treatment of protesters.

The paper, Al Wasat, was suspended on 2 April over charges that it had falsified news, but resumed publishing the next day after its editor-in-chief Mansoor al-Jamri, its British managing editor Walid Noueihed and head of local news Aqeel Mirza agreed to resign. On 4 April, two Iraqi journalists working for Al Wasat, Raheem al-Kaabi and Ali al-Sherify, were deported without trial.

Jamri, Noueihed and Mirza said they received a fax last week from the government's media arm, the information affairs authority, notifying them that they would be questioned by the public prosecutor over the alleged fabrication of news.

Jamri, who was questioned first, said he admitted to publishing six incorrect articles as accused. However, he argued that the false news was emailed to Al Wasat from the same IP address as part of an apparent campaign to plant disinformation.

He said this news slipped through the editing net as Al Wasat, whose printing press was attacked on 14 March and whose offices – inside the curfew zone imposed the same week – was operating on a skeleton staff.